The present invention relates to fiber optic sensing technology, and more specifically to a sensor and method using a Fabry-Perot optical interferometer and spectral signal demodulation for measurement of a physical parameter such as a pressure, a temperature, a strain, and a refractive index with a high accuracy and within a large range of physical parameters.
Fiber optic interferometric sensors offer a number of advantages over electrical sensors such as inherent immunity to electromagnetic interference, capability of operating in harsh environments and at longer distances, miniature sizes, etc. A fiber optic Fabry-Perot sensor typically consists of a measuring Fabry-Perot probe, a fiber optic extension cable and an opto-electronic module. The measuring Fabry-Perot probe represents a cavity with two partially reflective surfaces. The cavity modulates the spectrum of the incident light depending on cavity spacing and the refractive index of the media inside the cavity. Both, the cavity spacing and the refractive index define the optical path difference between the beams that are reflected from each reflective surface at certain wavelength. The optical path difference determines the conditions for maximums and minimums of the spectral modulation, which is changed with the physical parameter. The extension cable connects the probe to the opto-electronic module by means of transmitting the light from the light source to the probe and transmitting the modulated light back to the opto-electronic module for demodulation and further signal processing. The opto-electronic module includes a light source and a fiber optic coupling means for connecting the light source to the extension cable. The probe can be connected directly to the opto-electronic module if a long distance is not required.
Light sources which produce a broad spectrum light (polychromatic or white light sources) are cheap and less susceptible to thermal instability. They are preferred to be used in fiber optic sensors for wide industrial applications where the typical operating temperature is ranging from −40 C to +60 C. The returning light is demodulated in the opto-electronic module either by means of a second interferometer or by spectral means. Interferometric demodulation provides low signal-to-noise ratio unless mechanical scanning of the reference mirror is provided. Precise mechanical scanning may be done only in a stabilized environment which is costly to achieve and maintain. Spectral demodulation does not require mechanical scanning. It can be achieved by using conventional microspectrometers which have miniature size and robust design. Fiber optic spectroscopes are primarily based on diffractive gratings which provide linear spectra along the wavelengths. Modern diffractive gratings, in particular, holographic diffractive gratings have high efficiency and resolution along wide range of spectrum.
A number of techniques have been proposed for improving fiber optic interferometric sensors with spectral demodulation. U.S. Pat. No. 6,577,402, MILLER, Jun. 10, 2003 discloses a fiber optic interferometric sensor with spectral demodulation which is based on comparison of spectral light intensities taken at two wavelengths with the reference values recorded for the same wavelengths. The wavelengths are preferably selected at zero crossing locations where the spectral modulation has the highest contrast. Such a technique may be applied only for sensing the physical parameter within a narrow range. This range is limited by a half-period of the spectral modulation, and if the modulated optical spectrum shifts beyond its reference location, the two-point technique will indicate the value of the physical parameter which corresponds to the shift calculated from another fringe of the modulated spectrum.
A technique described in U.S. Pat. No. 6,141,098, SAWATARI et al., Oct. 31, 2000 is based on recording a calibration set of modulated spectra and comparing the measured spectrum with the calibration set. The modulated spectrum is normalized by subtracting the reference fringe pattern from the actual data fringe pattern, and then dividing by the average intensity of the reference pattern. However, such way of normalization requires both measuring and calibration conditions to be identical, which is difficult to achieve in practice. The measuring spectrum is changed with the fiber length, and also, from one opto-electronic module to another because of the spectral variability of the light sources. The deviation of the real measuring condition from the calibration condition, which is seen from the fluctuation of the average value in the referred document, leads to the higher inaccuracy because the measuring spectrum could not match with any stored calibrated data.
Another fiber optic interferometric sensor utilizing spectral decoding is described in U.S. Pat. No. 4,945,230, SAASKI et al., Jul. 31, 1990 and related documents. The sensor is based on a Fabry-Perot sensing interferometer with a very short optical path (two mirrors in a Fabry-Perot cavity are located closely). The interferometric pattern in such an arrangement has only one minimum which shifts with the physical parameter. The position of the interferometric minimum changes the proportion of light from each side of the minimum which can be registered by two photodetectors. Although simple, this method also has limited measuring range because the physical parameter can be measured within the fraction of a single fringe.
Also known in the art is the document TAPIA-MERCADO, et al., “Precision and sensitivity optimization for white-light interferometric fiber optic sensor”, J. Lightwave Technol., v. 19, 2001, pp. 70–74 describing a zero-crossing technique in Sagnac fiber optic temperature sensors. The procedure of analyzing the shift in the interferometric pattern which includes determining the position of zero crossing points is based on normalization of the recorded spectra by calculating the two auxiliary spectra which are found by interpolating the maxima and minima of smoothed recorded spectrum. However, the interpolation error is increased with the smaller number of fringes in the spectrum and, consequently, the accuracy of the determination of the zero-crossing points is reduced.
Yet known in the art is the document EGOROV et al., “High reliable, self calibrated signal processing method for interferometric fiber-optic sensors”, SPIE, v. 2594, 1996, pp. 193–197 describing a method for signal demodulation. The method is based on converting the optical spectrum into the optical frequency domain, calculating the Fourier transform of the converted signal, filtering the oscillating component, calculating the inverse Fourier transform and calculating a derivative of the final phase spectrum which gives the absolute value of the optical path difference. The conversion of the spectra from wavelength to optical frequency domain requires a long computation time because it must include a number of splining operations in order to achieve acceptable accuracy. This complexity increases the response time of the sensor making it too slow for sensing some physical parameters. The algorithm must include tracking of the oscillating frequency which is changed with the physical parameter. The value of the oscillating frequency is determined as a maximum in the amplitude part of the first Fourier transform which adds yet another complexity to the method and increases the response time of the sensor. No normalization has been proposed for this method based on assumption that spectral change of the light source will not affect the phase distribution.
Many applications require not only tracking the value of the sensing parameter, but measurement of the physical parameter in a large range. Such a requirement is typical of absolute temperature and pressure measurements for downhole applications in the oil and gas industry, or the measurement of strain in construction, etc.
Typically fiber optic sensors are installed in locations that are different from those where sensors were calibrated. The difference occurs due to the light attenuation in the extension cable, which can vary in length from one installation to another. Bending of the fiber affects the mode content which changes the intensity of the light coming to the opto-electronic module. Polychromatic light sources, such as LED's, in particular, usually have spectral power distributions that vary from unit to unit. Replacing the opto-electronic module requires recalibration of the probe. This is a complex task because it affects the sensing environment. For example, the recalibration of the pressure probe requires a depressurization of the pressurized vessel, which is associated with the interruption of the technological process. Recalibration is impossible in fiber optic strain sensors, which are used in construction because probes are permanently embedded into the concrete. Light sources are subjects to degradation with time and the aging reduces the total amount of light coming to the spectroscope, consequently causing the fluctuation of the average value. The latter introduces a systematic error in zero-crossing algorithms. Therefore, there is a need for a method and a sensor with a reliable normalization technique.
An object of the present invention is to provide a fiber optic sensing device based on white light interferometric method with spectral demodulation of the interferometric spectrum, for measuring a physical parameter within a range that is not limited by a single fringe of the modulated spectrum.
It is another object of the invention to provide such a fiber optic sensing device with a high accuracy and stability.
It is a further object of the invention to provide such a fiber optic sensing device with a possibility for in-field normalization of the signal and means for correcting the variation of spectral intensity of light caused by the bending of fibers and instability of the light source.